Wrong WUR in '81 SC - Should I swap it?

[mike635]

Looking for an opinion on whether or not I should replace my incorrect (was replaced by the previous owner) warm up regulator with the correct part. Here's the story:

I’ve been working with the CIS system this summer to get it adjusted properly (just the basics, no real problems other than somewhat high fuel consumption) The car now runs/starts flawlessly, and I’ve got the mixture close to correct (according to the frequency valve duty cycle = 50% fluctuating happily) and recently checked and got 19 MPG city/stop/go driving which is quite good for an ‘81 with 155K miles. I haven't checked the CO (no meter) but assume it's reasonable given the MPG and performance.

I checked my cold and warm fuel pressure and found that the warm pressure is out of spec, it only rises to 3.1 bar (should be 3.5 according to Bentley). The WUR was replaced to resolve a hot start issue but I never really bothered to look at it until now. I discovered that they installed a 009 WUR (which I believe is correct for a ’74 911). Looking up the specs for the 009, I think 3.1 bar is correct for this part however my ’81 SC (USA) requires the 090 regulator.

Would make sense to replace the regulator with the correct part? I'm tempted to leave it alone but I like things to be "right" whenever possible and I know the warm fuel pressure is important because it effects the mixture. I do run a cat and have a newer O2 sensor.

Mike

[MotoSook]

The car now runs/starts flawlessly

You like that statement? if yes, leave it alone.

The pressure is a range. Look at your P -vs- T chart...it's a range. Accuracy of measurement (pressure and temperature) leaves plenty of margin.....you want more precision? Go to EFI...otherwise go enjoy the car!

[Joe Bob]

If it werks....screw it...

[psalt]

Mike,

The symptom of +10% lower system pressure on an 81 SC would be high load, open loop leanness. If the engine does not ping under WOT @4500 and the plugs are not too white, it may not be an issue. My experience is the opposite, that the later lambda SC engines run lean in open loop and WOT, and they work best with the mixture set to fluctuate at 20-30 dwell rather than 45-55. My advice would be to beg, borrow or steal a wide band O2 sensor. If the AFR under WOT load is above 13.5, then I would adjust the mixture, retest, only then replace the WUR.

Paul

[mike635]

Thanks everyone.

Paul - I've thought about getting a wide band O2 sensor in the past, maybe now is the time. I've read (and re-read) many of your posts regarding CIS troubleshooting and learned a lot including some theory and how to monitor and adjust the system using the diagnostic port and a scope.

I think I'll leave the regulator as is for now and look around for a wideband sensor kit.

Mike